A lead storage battery (particularly, a control valve type lead storage battery) has been increasingly used as a drive source for an electric vehicle, other than for starting up an engine of a vehicle or for a backup power source. It is desirable to charge a lead storage battery rapidly in a short time, in view of a user's demand for constantly setting an electric vehicle in a driving ready state. In particular, in a vehicle for commercial use, there is a demand for charging the vehicle during a break time of the user.
In view of the above, there is known a charging method of charging a lead storage battery with a constant current, and changing a charging current value to be stepwise decremented, each time a terminal voltage of the lead storage battery reaches a predetermined threshold voltage. Such a charging method is known as a n-stage constant current charging method, because the charging current value is changed to be stepwise decremented (n−1) times (where n is an integer of 2 or larger), and charging is performed with current values of n-stages. The n-stage constant current charging method is known as a lead storage battery charging method that enables to obtain a large charging quantity of electricity in a short time (see e.g. patent literature 1).
Here, a lead storage battery is known to have characteristics that the higher the temperature is, the higher the charging efficiency is, and the lower the temperature is, the lower the charging efficiency is. Accordingly, if the lead storage battery is charged in the same manner without depending on a temperature, deficient charging is performed in a low-temperature condition, and overcharging is performed in a high-temperature condition. In view of this, patent literature 1 discloses an idea of performing proper charging of a lead storage battery without excess or deficiency by adjusting a second-stage (last stage) charging time depending on a temperature of the battery when charging is switched from first-stage charging to second-stage charging.
However, it is difficult to match the state of charge (hereinafter, called as SOC) of a lead storage battery to a fully charged state of 100% (a state where the dischargeable quantity of electricity is equal to a nominal capacity value) within a user's break time of about 10 to 60 minutes during work. Accordingly, charging is ended before the lead storage battery is brought to a fully charged state. Hereinafter, charging which is interrupted or ended before a fully charged state is reached is called as deficient charging. A lead storage battery used as a drive source for an electric vehicle repeats deficient charging.
Here, SOC represents a ratio of charged quantity of electricity to a full charging capacity of a battery in the unit of percentage (%).
If deficient charging and discharging are repeated, lead sulfate as a reaction product of discharging deposits on a positive electrode and on a negative electrode of the battery, namely, a degradation mode is generated. Such a degradation mode is called as sulfation. In order to eliminate the degradation, there is proposed an idea of performing refresh charging to eliminate sulfation by raising the SOC which is controllably set in the range of from about 50 to 70% in a normal operation condition to 100% i.e. by achieving a fully charged state (see e.g. patent literature 2). Specifically, patent literature 2 discloses that stepwise incrementing the SOC of a control valve type lead storage battery toward 100% is advantageous than instantaneously raising the SOC from a state where the SOC is kept to 70% to 100%.
However, the inventors of the present application found that sulfation cannot be sufficiently eliminated by setting the SOC of a lead storage battery to 100% as described above i.e. by refresh charging of performing proper charging without excess or deficiency, in the case where sulfation progresses resulting from repeating deficient charging and discharging as described above.